[Dúvida] O meu SnackBar não esta dando certo

ScaffoldMessenger.showSnackBar was called, but there are currently no descendant Scaffolds to present to. 'package:flutter/src/material/scaffold.dart': Failed assertion: line 289 pos 7: '_scaffolds.isNotEmpty'

fica dando este erro

Meu Codigo: import 'package:flutter/material.dart';

class FormScreen extends StatefulWidget { const FormScreen({Key? key}) : super(key: key);

@override State createState() => _FormScreenState(); }

class _FormScreenState extends State { TextEditingController usernameController = TextEditingController(); TextEditingController passwordController = TextEditingController(); final _formKey = GlobalKey();

@override Widget build(BuildContext context) { return MaterialApp( home: Scaffold( appBar: AppBar( title: Text('Login'), ), body: Form( key: _formKey, child: Center( child: SingleChildScrollView( child: Container( height: 650, width: 375, decoration: BoxDecoration( color: Colors.black12, border: Border.all(color: Colors.black, width: 2), borderRadius: BorderRadius.circular(8), ), child: Column( children: [ Padding( padding: const EdgeInsets.all(8.0), child: TextFormField(

                      textAlign: TextAlign.center,
                      controller: usernameController,
                      validator: (value) {
                        if (value!.isEmpty && value != 'Victor') {
                          return 'Username Errado';
                        }
                        return null;
                      },
                      decoration: InputDecoration(
                          border: OutlineInputBorder(),
                          hintText: 'Username',
                          fillColor: Colors.white70,
                          filled: true
                      ),
                    ),
                  ),
                  Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: TextFormField(
                      textAlign: TextAlign.center,
                      controller: passwordController,
                      validator: (value) {
                        if (value!.isEmpty && value != '123') {
                          return 'Username Errado';
                        }
                        return null;
                      },
                      decoration: InputDecoration(
                          border: OutlineInputBorder(),
                          hintText: 'Passoword',
                          fillColor: Colors.white70,
                          filled: true),
                    ),
                  ),
                  ElevatedButton(
                      onPressed: () {
                        if (_formKey.currentState!.validate()) {
                          ScaffoldMessenger.of(context).showSnackBar(
                            const SnackBar(content: Text('Teste'))
                          );
                        }
                      },
                      child: Text('Adicionar'))
                ],
              ),
            ),
          ),
        ),
      )),
);

} }